Differential control gas lift system

ABSTRACT

A well bore gas lift system for elevating well bore liquids through a tubing string to the surface of the well bore. A main pressure differential valve controls the feed of gas from the tubing-casing annulus into the tubing string, the main valve containing a pilot pressure differential valve that also controls the gas feed from the tubing-casing annulus into the tubing string. The pilot or normal high differential pressure valve opens to admit gas to the tubing string on reduction of the casing-to-tubing differential pressure. This action further reduces the casing-to-tubing differential pressure, causing opening of the main valve, both valves then being open. The pilot valve first opens, assisting opening of the main valve. The main valve first closes, assisting closing of the pilot valve.

United States Patent 3,336,391 6/1968 Garrett Primary Examiner-Carlt0n R. Croyle Assistant Examiner-R. E. Gluck Attorney-Bernard Kriegel ABSTRACT: A well bore gas lift system for elevating well bore liquids through a tubing string to the surface of the well bore. A main pressure differential valve controls the feed of gas from the tubing-casing annulus into the tubing string, the main valve containing a pilot pressure differential valve that also controls the gas feed from the tubing-casing annulus into the tubing string. The pilot or normal high differential pressure valve opens to admit gas to the tubing string on reduction of the casing-to-tubing differential pressure. This action further reduces the casing-to-tubing differential pressure, causing opening of the main valve, both valves then being open. The pilot valve first opens, assisting opening of the main valve. The main valve first closes, assisting closing of the pilot valve.

DIFFERENTIAL CONTROL GAS LIFT SYSTEM The present invention relates to subsurface well bore apparatus. and more particularly to a differential gas lift system used to inject relatively high-pressure gaseous fluid into a tubing string disposed in a well bore. for the purpose of elevating the liquid in the tubing string to the surface or top of the well bore.

Under some well conditions, gas lift equipment used for elevating liquid through the tubing string to the top of the well bore does not operate properly or efficiently. The gas lift valve may open at the desired pressure differential between the gas pressure in the tubing-casing annulus and the pressure in the tubing string, but the gas lift valve does not close upon the arrival and discharge of the slug of liquid at the top of the well bore. For the gas lift valve to close. the tubing pressure must reduce to a certain level below the tubing-casing annulus pressure to provide a required valve closing differential pressure. The failure of the valve closing differential pressure to be realized may also present itself where the tubing-casing annulus pressure is reduced excessively, as by low deliverability of gas into the annulus. by the presence of a small-diameter casing in the well bore, or by borderline supply gas pressure.

It is desirable for a large port area to be present in the differential valve mechanism through which the gas under pressure can flow, in order for efficient ballistic lifting to occur. Such large port area provides a high flow rate of gas, and where such high flow rate is coupled with the presence of wet gas in the tubing-casing annulus, and further with the necessity for lifting the liquid through comparatively long tubing strings. valve closure is prevented after the desired quantity of the liquid has been discharged at the top of the well bore.

By virtue of the present invention, a large gas lift valve port area is provided through which the gas can enter the tubing string for the purpose of creating high flow of gas for efficient lift of the slug of liquid in the tubing string to the top of the well bore. Despite the presence of such large port areas. and even in the presence of wet lifting gas and long tubing strings, the valve device will still close at the required valve-closing differential. This will occur despite the fact that the large rate of gas flow through the tubing string will increase the pressure in the tubing string to the point at which the gas lift valve mechanism would otherwise remain in an undesired open condition.

More specifically, the invention contemplates the provision of main and pilot gas lift valves at the gas-injecting point in the tubing string, which may consist of two differential gas lift valves. The pilot gas lift valve is set to operate at a normal, comparatively high pressure differential, while the main gas lift valve is set to operate at a very low pressure differential; that is to say. the high-set pressure differential pilot valve will open and will close when the tubing pressure is at a certain valve compared to the tubing-casing annulus pressure; whereas. the very low differential pressure or main valve will open and will also close when the tubing pressure is at a much higher pressure. The high-set differential pilot gas lift valve will first open to allow gas to flow into the tubing string, such flow of gas elevating the pressure in the tubing string to the point at which the low-set differential main gas lift valve will open. so that the gas flowing therethrough will combine with the gas flowing through the high-set differential pilot valve and assist the elevation of the slug of liquid in the tubing string to the top of the well bore. When the pressure in the tubing string drops to a certain value. the low set pressure differential main valve will close. but the high-set pressure dilferential pilot valve will remain open. The closing of the low-set pressure dif ferential main valve will reduce the flow of gas into the tubing string and will reduce the pressure therewithin to the point at which the high set pressure differential pilot valve will also close. Thus. the highset pressure differential pilot valve assists the low-set pressure differential main valve to open; whereas. the low-set pressure differential main valve assists the high-set pressure differential pilot valve to close. The two differential valves provide a large combined port area with improved operating characteristics to secure efficient ballistic lift of the slug of liquid in the casing string. and such improved operating characteristics of the system are of particular advantage in wells where wet gas must be used for effecting lifting of the slug in long tubing strings. it is also effective where the tubingcasing annulus pressure may drop because of low deliverability of gas thereto.

This invention possesses many other advantages. and has other purposes which may be made more clearly apparent from a consideration of a form in which it may be embodied. This form is shown in the drawings accompanying and forming part of the present specification. It will now be described in detail, for the purpose of illustrating the general principles of the invention; but it is to be understood that such detailed description is not to be taken in a limiting sense. since the scope of the invention is best defined by the appended claims.

Referring to the drawings:

FIG. I is a diagrammatic view of a differential gas lift system embodying the invention;

FIGv 2 is a longitudinal section through the pilot and main gas lift valves embodied in the system disclosed in FIG. 1 and disposed in closed condition;

FIG. 3 is a view similar to FIG. 2, with the pilot valve in open condition; and

FIG. 4 is a view similar to FIG. 2. with both the pilot and main valves in open condition.

The differential gas lift system disclosed in the drawings as illustrative of the invention is presented diagrammatically in FIG. 1. An oil well W has a casing string B therein. with fluid from a deep storage well C capable of passing through casing perforations D to the casing interior. A well packer E may be set in the casing string above the perforations and a tubing string T is disposed in leakproof relation to the packer so that the well fluid can flow upwardly through the tubing string to the top of the well bore. the tubing string extending in sealed relation through a casing head G and into a production line H under the control of a surface valve 1. In some instances. the wall packer need not be used. the gas from the storage well C passing into the casing-tubing annulus K and to the top of the well bore. A suitable mechanism (not shown) is located in the tubing string T to control the flow of water from the well C into the tubing string T. this water being removed when it reaches a certain elevation in the tubing string. as described hereinbelow.

As specifically disclosed in the drawings, main and pilot gas lift valves R, S are mounted on the tubing string closely adjacent to the well packer E, these gas lift valves controlling the injection of gas from the casing-tubing annulus K into the tubing string. the gas being supplied from a suitable compressor or supply line through the gas line L into the upper portion of the casing string. there being a suitable valve M for controlling such flow of gas. The pilot gas lift valve S is set to open at a relatively high casing annulus-tubing pressure differential; whereas. the main gas lift valve R is set to open at a substantially lower casing annulus-tubing pressure differential.

The differential gas lift valve mechanism includes a fitting 20 suitably secured. as by welding, to the exterior of the tubing string and having its internal passage 21 communicating with the interior of the tubing string T through a port slot 22. The lower portion 23 of the fitting is internally threaded for threaded attachment of a differential gas lift housing 24 thereto, this housing having a cylindrical valve seat 27 adapted to the closed by the upper valve head portion 28 of a main piston valve member 29 slidably mounted in the inner bore of the housing below a plurality of side chokes or orifices 30 opening through the wall of the housing 24 below the valve seat 27. Upward movement of the piston valve member 29 is limited by engagement of its shoulder 31 with a companion downwardly facing shoulder 32 on the housing. at which time the valve head 28 will be disposed within the cylindrical seat 27. leakage of fluid between the head and seat being prevented by a suitable side seal ring 33 mounted in the head and sealingly engaging the valve seat 27, When the valve head is sealingly engaged with the seat, the fluid pressure in the tubing string can act in a downward direction over the area ofthe valve head.

The main piston valve member 29 is shiftable downwardly to a position in which its valve head 28 is completely removed from the valve seat 27, opening the chokes 30 to the passage of gas from the tubing-casing annulus K through the valve seat 127 and into the tubing string T. Downward movement of the piston valve is limited by engagement of a downwardly facing piston shoulder 35 with a suitable stop ring 36, in the form ofa split snap ring, mounted within an internal groove 37 in the lower portion ofthe valve housing 24. Such downward shifting of the valve head occurs as a result of the combined action of the fluid pressure in the tubing string acting downwardly over the area of the piston valve 29 and a tension spring 38, the upper end of which is connected to the piston valve 29 through a pilot member 60 movable in the main valve member 29, the lower end of the spring being suitably connected to a spring anchor member 39 welded, or otherwise attached, to the exterior of the tubing string T, the string being protected by a suitable enclosure 40. When the piston valve member 29 is in its downward or full port opening position, the spring 38 may have very little tension in it.

As stated above, a pilot piston valve member 60 is embodied in the main valve member 29, the latter functioning as a housing for the pilot piston valve member. The main valve member has 21 upper passage 6] therethrough providing a cylindrical valve seat 62 adapted to be closed by the upper valve head portion 63 of the pilot piston valve member, which is slidably mounted in the inner bore 64 of the main valve member below a plurality of side chokes or orifices 65 opening through the wall of the main valve member 29 below its cylindrical valve seat 62. Upward movement of the pilot piston valve member 60 is limited by engagement of its shoulder 66 with a companion downwardly facing shoulder 67 on the main valve member, at which time the valve head 63 will be disposed within the cylindrical seat 62, leakage of fluid between the head and seat being prevented by a suitable side seal ring 68 mounted in the head and sealingly engaging the valve seat. When the pilot valve head 63 is sealingly engaged with the seat 62, the fluid pressure in the tubing string T can act in a downward direction over the area of the pilot valve head.

The pilot piston valve member 60 is shiftable downwardly to a position in which its valve head 63 is completely removed from the valve seat 62, opening the chokes 65 to the passage of gas from the tubing-casing annulus K which flows through side ports 30, 69 in the housing 24, through the chokes 65 and through the valve seat 62 into the tubing string T. Downward movement of the pilot piston valve member is limited by engagement of a downwardly facing piston shoulder 70 with a suitable stop ring 71, in the form ofa split snap ring, mounted within an internal groove 72 in the lower portion of the main valve member. Such downward shifting of the valve member 60 occurs as a result of the combined action of the fluid pressure in the tubing string T acting downwardly over the area of the pilot piston valve and the tension spring 38. The upper end of this tension spring is connected to a depending stern portion 73 of the pilot piston valve member, its lower end, as described above, being connected to the spring anchor member 39. The spring is tending to pull the pilot piston valve member 60 downwardly to an open position until it engages its stop ring 7l, whereupon the spring exerts a force on the main piston valve member 29 tending to urge it to a position removing its head 28 from the cylindrical valve seat 27 of the housing 24. for the purpose of opening the chokes or orifices 30 in the housing to the passage of fluid from the tubing-casing annulus K into the seat 27 and tubing string T.

Assuming the pilot valve member 60 and the main valve member 29 to both be in the closed position, as disclosed in FIG. 2, the gas pressure P, in the tubing-casing annulus K acts in an upward direction over the area A of the pilot piston valve, which area is much greater than the area A, ofthe upper valve head portion 63 of the pilot piston valve. When the pilot piston valve is closed, the gas pressure P is also acting in a downward direction over the annular area A, of the largediameter portion 75 of the pilot piston valve, which is the area between the cylindrical valve seat 62 and a larger diameter surface of h the cylindrical portion 64 of the main valve member 29 in which the pilot piston slides. This pressure is designated P, in the drawings (FIG. 4). When the pilot valve S is in the closed condition, the gas pressure can still pass through the chokes 30 to the interior of the main valve member 29 above the piston shoulder having the area A, and act downwardly on the pilot piston valve 60. The gas pressure acting over this area A, makes the resultant area over which the gas pressure acts, with the valve S in the closed condition. the same area A, as the area A, of the seat of the pilot portion ofthe gas lift valve; that is. t,,--A,,=A,

Assuming both the main and pilot valves R, S to be in the closed condition, the gas pressure in the tubing-casing annulus K acts in an upward direction over the area A of the pilot and main valves 5, R, which area is much greater than the area A of the upper valve head portion 28 ofthe main piston valve 29. When the main valve R is closed, the gas pressure is also acting in a downward direction over the annular area A of the large-diameter piston portion, which is the area between the cylindrical valve seat 27 and the larger diameter surface of the cylindrical portion 42 of the housing in which the main piston 29 slides. When the main valve R is in the closed condition, the gas pressure can still pass through the chokes 30 to the interior of the housing 24 above the piston shoulder having the area A and act downwardly on the main piston valve. The gas pressure acting over this area .4 makes the resultant area over which the gas pressure acts, with the main and pilot valves in the closed condition, the same area A as the area 4,, ofthe seat of the gas lift valve; that is, A,,,,-A,,=A,,

Considering first the pilot valve S, when it is in the closed condition, the tubing pressure P, is acting downwardly over the area A tending to shift the piston valve to open posi' tion. The gas pressure P in the tubing-casing annulus K is acting upwardly over the area A,,A,. or A,-, tending to maintain the pilot piston valve in the closed condition. The spring 38 is pulling the pilot piston valve 60 in a downward direction, supplementing the force of the tubing pressure P, and tending to shift the valve S to open position. When the liquid level in the tubing string T has risen sufticiently, so that its downward force on the pilot piston valve 60, supplemented by the spring force, exceeds the force of the gas pressure P,. acting upwardly on the pilot piston valve 60, the latter will be shifted downwardly to an open condition, as disclosed in FIG. 3, and the gas under pressure can then flow through the open ports or chokes and through the valve seat 62, continuing through the tubing port 22 to the interior of the tubing string, commencing the lifting of the slug or column of liquid ahead of it to the top of the well bore. The gas pressure P, within the main valve 29 on the downstream side of the orifices 65, which will be less than the gas pressure P, in the tubing-casing annulus K, because of the throttling action of the orifices 65, will then be acting in a downward direction on the pilot piston valve 60 over its full area A,,, this force being supplemented by the force of the spring 38 to maintain the pilot piston valve member 60 in the open condition. As the slug of liquid is discharged at the top of the well bore, the flow of gas through the orifices 65 accelerates, the gas pressure P, on the downstream side of the orifices 65 and acting downwardly on the pilot piston valve 60 decreasing, until it and the spring 38 are insufficient to hold the pilot valve 60 in the downward or valve opening condition, the greater gas pressure P, in the tubing-casing annulus K acting upwardly over the area A, of the piston valve and shifting it to the closed condition.

By making the area A, over which the gas pressure P, acts, with the valve in the open condition, greater than the area A, over which the tubing pressure P, acts, when the valve 60 is in the closed condition, the area A of the orifices or chokes can be made much greater. since the large piston area A compensates for the smaller pressure drop created by the enlarged chokes or orifices 65 of area A,. There is thus provided a differential valve that will open and close at approximately the same differential pressure.

The main valve R functions in essentially the same manner as the pilot valve S. ASsuming the pilot piston valve 60 to have shifted to its opened condition, such as disclosed in FIG. 3, and with the main valve R still closed, the gas pressure in the tubing-casing annulus K is acting upwardly over the main piston valve, tending to maintain it in the closed condition. The spring 38 is now acting on the main piston valve 29 in a downward direction, since the pilot piston valve 60 has engaged the stop ring 71, and is supplementing the force of the tubing pressure acting downwardly on the main piston valve and tending to shift the latter downwardly to open position. When the pressure in the tubing string has risen sufficiently, as described hereinbelow, so that its downward force on the main piston valve 29, supplemented by the spring force, exceeds the force of the gas pressure acting upwardly on the main piston valve, the latter will be shifted downwardly to an open condition (FIG. 4), and the gas under pressure can then flow through the open housing ports 30, through the housing valve seat 27, and through the tubing port 22 to the interior of the tubing string T, to assist the gas flowing through the open pilot valve portion S of the apparatus in lifting the slug or column of liquid ahead of it to the top of the well bore. As in connection with the pilot piston valve, the gas pressure P,, within the valve housing 24 on the downstream side of the orifices 30, which will be less than the gas pressure P,. in the tubing-casing annulus K, because of the throttling action of the orifices 30, will then be acting in a downward direction on the piston valve 29, this force being supplemented by the force of the spring 38, which, at this time, can be rather low to maintain the main piston valve member in the open condition. However, as the slug of liquid is discharged at the top of the well bore, the flow of gas through the orifices 30 accelerates, the gas pressure I" on the downstream side of the orifices and acting downwardly on the main piston valve 29 decreasing until it and the spring 38 are insufficient to hold the main valve R in an open condition, the greater gas pressure P, in the tubing-casing annulus K shifting the main piston valve 29 to its closed condition.

The pilot and main gas lift valves 5, R disclosed have a very desirable characteristic of large orifice ports 65, 30 disposed therethrough to permit a large flow rate of gas into the tubing string T when the valves are in the open position, which results in efficient ballistic lift of the liquid slug in the tubing string to the top of the well bore. Such large orifice ports can be used because of the making of the areas of the piston valve members 29, 60 over which the tubinG-casing annulus gas pressure P, acts greater than the area over which the tubing pressure P, acts. In some well installations, the tubing pressure P, will not drop sufficiently to close a single differential valve, since the tubing pressure does not reduce to the point at which the differential pressure increases to the valve-closing condition as the slug of fluid is discharged at the surface of the well bore. By virtue of the present invention, the difficulty is solved in that a large port area is provided to allow high gas rates offlow into the tubing string, while assurance is had that each gas lift valve will close at the required valve-closing differential pressure.

In place of a single differential gas lift valve with a large port area being used, a plurality of differential valves, such as the main valve R and pilot valve 5, are employed as disclosed in the drawings. the total desired orifice port area is divided between the main valve R and the pilot valve 8. As an example. the area A through the orifices 65 of the pilot valve may be about one-third of the total required area; whereas, the area A, of the orifice ports 30 of the main valve are about two-thirds of the required total area. Thus, the two valves. when open, act together to provide the desired large total orifice area through which the gas is injected into the tubing string for efficient ballistic lift. The pilot difi'erential gas lift valve S is set to open and close at a relatively high pressure differential; whereas, the main gas lift valve R is set to operate at a lower pressure differential. As the liquid level in the tubing string T rises, with both gas lift valves closed, the high-set dif ferential pilot valve S will open first (FIG. 3), and such opening pressure may be at the desired tubing pressure P, buildup. When the high-set differential pilot valve S opens, gas will flow through its ports 65 and into the tubing string T to begin elevating the slug ofliquid in the tubing string, but, at the same time, also increasing the fluid pressure P, in the tubing string, causing the annulus-tubing pressure differential to decrease below the setting of the low-set differential main gas lift valve R, such main valve then opening (FIG. 4). As a result, both the pilot and main gas lift valves 5, R are now open and the tubing-casing annulus gas is passing through the combined areas of the ports or orifices of the two valves into the tubing string for efficiently lifting the slug of fluid in the tubing to the top of the well bore. During discharge of the liquid slug at the top of the well bore, the tubing pressure P, progressively decreases, and when it reaches a value below the tubing-casing annulus pressure P, that provides the required differential for closing the low-set differential main valve R, such valve will close. The gas can then flow only through the high-set differential pilot gas lift valve S through its ports 65 of a lesser area than the combined areas of both differential gas lift valves, not only reducing the gas injected through the high-set differential pilot gas lift valve 5, but also the pressure in the tubing string T. When such pressure reduces sufficiently, the pilot gas lift valve S will close.

The foregoing cycle of operation is repeated intermittently. Assuming both valves to be closed, the high-set differential pilot gas lift valve S will first open, and such opening will effect opening of the low-set differential main valve R. When the tubing pressure is decreased as a result of discharge of the slug of liquid at the top of the well bore, the low-set differential main valve R first closes, and such closing effects a further reduction of the pressure in the tubing string to the point at which the high-set differential pilot valve S closes. Thus, the high-set differential pilot valve S opens and triggers the lowset differential main valve R to open, the low-set differential main valve R first closing and triggering the high-set differential pilot valve 5 to close.

in the event that well conditions are such that back flow of fluids from the tubing string T into the tubing-casing annulus K is to be prevented, a check or one-way valve arrangement is provided in the housing. As specifically disclosed, a ball check valve element is shiftable downwardly into engagement with an upwardly facing valve seat 8! in the housing when both the pilot and main valves are in the open condition, and in the event that the pressure P, within the tubing string is greater than the pressure in the tubing-casing annulus. However, when both valves R and S are closed, as illustrated in FIG. 2, the ball valve element 80 rests upon the upper end of the upper valve head portion 28 of the piston valve member 29. This head portion has a bypass notch or passageway 82 formed therein extending downwardly to a small extent from its upper end, so that, regardless of the fact that the ball valve element is engaging the upper end of the valve head 28, the tubing pressure can still pass through the notch 82 into the valve seat 62 for action upon the valve head portion 63 of the pilot piston valve member 60.

With the main valve in the closed condition, as disclosed in FIGS. 2 and 3, the ball valve member 80 is prevented from engaging the valve seat 81, so that the tubing pressure can still act in a downward direction on the main valve head portion 28, urging the main piston valve member to its open condition in concert with the spring 38, after the pilot piston valve member 60 has engaged the stop ring 71. With the pilot piston valve open, as disclosed in FIG. 3, the gas under pressure can flow through the ports 30, 65 and upwardly through the valve seat 62, elevating the ball valve member 80 within the housing 24 and the fitting the port slot 22 being too narrow to permit passage of the ball valve element into the tubing string.

The ball valve element will be in essentially the same condition when both valves are open, as illustrated in FIG. 4.

lclaim:

i. In apparatus for lifting fluid flowing from a subsurface zone into a well bore: a tubing string in the well bore extending to the top of the well bore and into which fluid from the zone can flow; a main gas lift valve connected to the tubing string for injecting gas from the well bore externally of the tubing string into the tubing string; a pilot gas lift valve in said main valve for injecting gas from the well bore externally ofthe tubing string into the tubing string; said main valve including a housing having a single central unobstructed passage and an inlet communicable with said passage, said housing having a valve seat surrounding said passage, a main valve member shiftable in said housing between a closed position engaging said seat and an open position disengaged from said seal and clear of said passage to fully open the same; said pilot valve having a central unobstructed pilot passage in said main valve member communicable with said single central passage, a pilot valve seat surrounding said pilot passage, and a pilot inlet communicable with said pilot passage, a pilot valve member shiftable is said main valve member between a closed position engaging said pilot seat and an open position disengaged from said pilot seat and clear of said pilot passage to fully open the same; said pilot valve being constructed and arranged to open at a desired normal difference in pressure between the fluid pressure in the well bore externally of the tubing string and the fluid pressure internally of the tubing string; said main valve being constructed and arranged to open at a pressure differential which is lower than the opening pressure differential of said pilot valve. 17

2. In apparatus as defined in claim I; said main valve being constructed and arranged to close at a desired normal difference in pressure between the fluid pressure in the well bore externally of the tubing string and the fluid pressure internally of the tubing string; said pilot valve being constructed and arranged to close at a pressure differential which is higher than the closing pressure differential of said main valve.

3. ln apparatus for lifting fluid flowing from a sub surface zone into a well bore: a tubing string in the well bore extending to the top of the well bore and into which fluid from the zone can flow; a main gas lift valve connected to the tubing string for injecting gas from the well bore externally of the tubing string into the tubing string, said main valve having a passage communicating with the interior of said tubing string and also having a valve seat surrounding said passage. means for conducting a gaseous fluid under pressure from the exterior of said main valve to said valve seat for flow therethrough into said passage, said conducting means including one or more orifices in said main valve on the upstream side of said seat, a main valve member shiftable in said main valve between a closed position engaging said seat and an open position disengaged from said seat; a pilot gas lift valve comprising a pilot passage in said main valve member communicating with the interior of said tubing string and also having a pilot valve seat surrounding said pilot passage. means for conducting a gaseous fluid under pressure from the exterior of said pilot valve to said pilot valve seat for flow therethrough into said pilot passage, said conducting means including one or more orifices in said pilot valve on the upstream side of said pilot seat. a pilot valve member shiftable in said main valve member between a closed position engaging said pilot seat and an open position disengaged from said pilot seat; said main and pilot valve members being subject to the pressure of the gaseous fluid in the well bore externally of the valves to shift said valve members to closed positions, each of said valve members also being subject to the pressure of the fluid in the tubing string when engaged with its valve seat tending to shift said valve members toward open position; and supplemental means exerting a force on said valve members tending to shift said valve members toward open position.

4. ln apparatus as defined in claim 3; said pilot gas lift valve being constructed and arranged to open at a desired normal difference in pressure between the fluid pressure in the well bore externally of the tubing string and the fluid pressure internally of the tubing string; said main valve being constructed and arranged to open at a pressure differential which is lower than the opening pressure differential of said pilot valve.

5. ln apparatus as defined in claim 3; said pilot gas lift valve being constructed and arranged to open at a desired normal difference in pressure between the fluid pressure in the well bore externally of the tubing string and the fluid pressure internally of the tubing string; said main valve being constructed and arranged to open at a pressure differential which is lower than the opening pressure differential of said pilot valve; said main valve being constructed and arranged to close at a desired normal difference in pressure between the fluid pressure in the well bore externally of the tubing string and the fluid pressure internally of the tubing string; said pilot valve being constructed and arranged to close at a pressure differential which is higher than the closing pressure differential of said main valve.

6. In apparatus as defined in claim 3; said one or more orifices in said pilot valve extending through said main valve member.

7. ln apparatus as defined in claim 3; each of said valve members having a first surface subject to the pressure of the gaseous fluid externally of the valves tending to shift said valve members toward its closed position, a second surface subject to the pressure of the fluid in the tubing string when the valve member is engaged with its seat tending to shift said valve member toward its open position. and a third surface subject to the pressure of the gaseous fluid in the valve between its valve seat and its associated one or more orifices tending to urge said valve member in a direction away from its valve seat. the area of said first surface and said third surface each being greater than the area of said second surface.

8. ln apparatus as defined in claim 3; each of said valve members having a first surface subject to the pressure of the gaseous fluid externally of the valves tending to shift said valve members toward its closed positions, a second surface subject to the pressure of the fluid in the tubing string when the valve member is engaged with its seat tending to shift said valve member toward its open position, and a third surface subject to the pressure of the gaseous fluid in the valve between its valve seat and its associated one or more orifices tending to urge said valve member in a direction away from its valve seat, the area of said first surface and third surface each being greater than the area of said second surface; said pilot gas lift valve being constructed and arranged to open at a desired normal difference in pressure between the fluid pressure in the well bore externally of the tubing string and the fluid pressure internally of the tubing string; said main valve being constructed and arranged to open at a pressure differential which is lower than the opening pressure differential of said pilot valve.

9. ln apparatus as defined in claim 3; each of said valve members having a first surface subject to the pressure of the gaseous fluid externally of the valves tending to shift said valve members toward its closed positions. a second surface subject to the pressure ofthe fluid in the tubing string when the valve member is engaged with its seat tending to shift said valve member toward its open position, and a third surface subject to the pressure of the gaseous fluid in the valve between its valve seat and its associated one or more orifices tending to urge said valve member in a direction away from its valve seat, the area of said first surface and said third surface each being greater than the area of said second surface; said pilot gas lift valve being constructed and arranged to open at a desired normal difference in pressure between the fluid pressure in the well bore externally of the tubing string and the fluid pressure internally of the tubing string; said main valve being constructed and arranged to open at a pressure differential which is lower than the opening pressure differential of said pilot valve; said main valve being constructed and arranged to close at a desired normal difference in pressure between the fluid pressure in the well bore externally of the tubing string and the fluid pressure internally of the tubing string; said pilot valve being constructed and arranged to close at a pressure differential which is higher than the closing pressure differential of said main valve.

III. In apparatus as defined in claim 3; wherein said supplemental means comprises spring means.

II. In apparatus as defined in claim 3; wherein said supplemental means comprises a spring connected to said pilot valve member, and coengageable means on said pilot valve member and main valve member for transmitting the force of said spring to said main valve member when said pilot valve member is in open position.

12. In apparatus as defined in claim 3; each of said valve members having a first surface subject to the pressure of the gaseous fluid externally of the valves tending to shift said valve members toward closed position. a second surface subject to the pressure of the fluid in the tubing string when the valve member is engaged with its seat tending to shift said valve member toward its open position, and a third surface subject to the pressure of the gaseous fluid in the valve between its valve seat and its associated one or more orifices tending to urge said valve member in a direction away from its valve seat, the area of said first surface and said third surface each being greater than the area of said second surface; said supplemental means comprising spring means.

I3. ln apparatus as defined in claim 3; each of said valve members having a first surface subject to the pressure of the gaseous fluid externally of the valves tending to shift said valve members toward closed position, a second surface subject to the pressure of the fluid in the tubing string when the valve member is engaged with its seat tending to shift said valve member toward its open position, and a third surface subject to the pressure of the gaseous fluid in the valve between its valve seat and its associated one or more orifices tending to urge said valve member in a direction away from its valve seat, the area of said first surface and said third surface each being greater than the area of said second surface; said supplemental means comprising a spring connected to said pilot valve member. and coengageable means on said pilot valve member and main valve member for transmitting the force of said spring to said main valve member when said pilot valve member is in open position.

14. In apparatus as defined in claim 3', said main valve having a check valve seat; a valve element engageable with said check valve seat to prevent fluid from flowing from said tubing string through said main valve passage to the exterior of said main valve; means on one of said valve members engaging said valve element to prevent its engagement with said check valve seat when said main valve member engages its companion seat; and means for enabling the pressure of fluid in said tubing string to act upon said pilot valve member when said pilot valve member engages said pilot seat.

15. In apparatus as defined in claim 3; said main valve having a check valve seat; a valve element engageable with said check valve seat to prevent fluid from flowing from said tubing string through said main valve passage to the exterior of said main valve; means on one of said valve members engaging said valve element to prevent its engagement with said check valve seat when said main valve member engages its companion seat; and means for enabling the pressure of fluid in said tubing string to act upon said pilot valve member when said pilot valve member engages said pilot seat, comprising a bypass passageway for fluid around said valve element.

16. In apparatus as defined in claim 3', said main valve having a check valve seat; a valve element engageable with said check valve seat to prevent fluid from flowing from said tubing string through said main valve passage to the exterior of of said main valve; means on one of said valve members engaging said valve element to prevent its engagement with said check valve seat when said main valve member engages its cornpanlon seat; and means for enabling the pressure of fluid in said tubing string to act upon said pilot valve member when said pilot valve member engages said pilot seat, comprising a bypass passage in said main valve member permitting flow of fluid around said valve element to said pilot valve member.

17. In apparatus as defined in claim 3; said main valve having a check valve seat; a valve element engageable with said check valve seat to prevent fluid from flowing from said tubing string through said main valve passage to the exterior of said main valve; means on said main valve member engaging said valve element to prevent its engagement with said check valve seat when said main valve member engages its companion seat; and means for enabling the pressure of fluid in said tubing string to act upon said pilot valve member when said pilot valve member engages said pilot seat.

18. In apparatus as defined in claim 3; said main valve having a check valve seat; a valve element engageable with said check valve seat to prevent fluid from flowing from said tubing string through said main valve passage to the exterior of said main valve; means on said main valve member engaging said valve element to prevent its engagement with said check valve seat when said main valve member engages its companion seat; and means for enabling the pressure of fluid in said tubing string to act upon said pilot valve member when said pilot valve member engages said pilot seat, comprising a bypass passage in said main valve member permitting flow of fluid around said valve element to said pilot valve member. 

1. In apparatus for lifting fluid flowing from a subsurface zone into a well bore: a tubing string in the well bore extending to the top of the well bore and into which fluid from the zone can flow; a main gas lift valve connected to the tubing string for injecting gas from the well bore externally of the tubing string into the tubing string; a pilot gas lift valve in said main valve for injecting gas from the well bore externally of the tubing string into the tubing string; said main valve including a housing having a single central unobstructed passage and an inlet communicable with said passage, said housing having a valve seat surrounding said passage, a main valve member shiftable in said housing between a closed position engaging said seat and an open position disengaged from said seat and clear of said passage to fully open the same; said pilot valve having a central unobstructed pilot passage in said main valve member communicable with said single central passage, a pilot valve seat surrounding said pilot passage, and a pilot inlet communicable with said pilot passage, a pilot valve member shiftable is said main valve member between a closed position engaging said pilot seat and an open position disengaged from said pilot seat and clear of said pilot passage to fully open the same; said pilot valve being constructed and arranged to open at a desired normal difference in pressure between the fluid pressure in the well bore externally of the tubing string and the fluid pressure internally of the tubing string; said main valve being constructed and arranged to open at a pressure differential which is lower than the opening pressure differential of said pilot valve. 17
 2. In apparatus as defined in claim l; said main valve being constructed and arranged to close at a desired normal difference in pressure between the fluid pressure in the well bore externally of the tubing string and the fluid pressure internally of the tubing string; said pilot valve being constructed and arranged to close at a pressure differential which is higher than the closing pressure differential of said main valve.
 3. In apparatus for lifting fluid flowing from a sub surface zone into a well bore: a tubing string in the well bore extending to the top of the well bore and into which fluid from the zone can flow; a main gas lift valve connected to the tubing string for injecting gas from the well bore externally of the tubing string into the tubing string, said main valve having a passage communicating with the interior of said tubing string and also having a valve seat surrounding said passage, means for conducting a gaseous fluid under pressure from the exterior of said main valve to said valve seat for flow therethrough into said passage, said conducting means including one or more orifices in said main valve on the upstream side of said seat, a main valve member shiftable in said main valve between a closed position engaging said seat and an open position disengaged from said seat; a pilot gas lift valve comprising a pilot passage in said main valve member communicating with the interior of said tubing string and also having a pilot valve seat surrounding said pilot passage, means for conducting a gaseous fluid under pressure from the exterior of said pilot valve to said pilot valve seat for flow therethrough into said pilot passage, said conducting means including one or more orifices in said pilot valve on the upstream side of sAid pilot seat, a pilot valve member shiftable in said main valve member between a closed position engaging said pilot seat and an open position disengaged from said pilot seat; said main and pilot valve members being subject to the pressure of the gaseous fluid in the well bore externally of the valves to shift said valve members to closed positions, each of said valve members also being subject to the pressure of the fluid in the tubing string when engaged with its valve seat tending to shift said valve members toward open position; and supplemental means exerting a force on said valve members tending to shift said valve members toward open position.
 4. In apparatus as defined in claim 3; said pilot gas lift valve being constructed and arranged to open at a desired normal difference in pressure between the fluid pressure in the well bore externally of the tubing string and the fluid pressure internally of the tubing string; said main valve being constructed and arranged to open at a pressure differential which is lower than the opening pressure differential of said pilot valve.
 5. In apparatus as defined in claim 3; said pilot gas lift valve being constructed and arranged to open at a desired normal difference in pressure between the fluid pressure in the well bore externally of the tubing string and the fluid pressure internally of the tubing string; said main valve being constructed and arranged to open at a pressure differential which is lower than the opening pressure differential of said pilot valve; said main valve being constructed and arranged to close at a desired normal difference in pressure between the fluid pressure in the well bore externally of the tubing string and the fluid pressure internally of the tubing string; said pilot valve being constructed and arranged to close at a pressure differential which is higher than the closing pressure differential of said main valve.
 6. In apparatus as defined in claim 3; said one or more orifices in said pilot valve extending through said main valve member.
 7. In apparatus as defined in claim 3; each of said valve members having a first surface subject to the pressure of the gaseous fluid externally of the valves tending to shift said valve members toward its closed position, a second surface subject to the pressure of the fluid in the tubing string when the valve member is engaged with its seat tending to shift said valve member toward its open position, and a third surface subject to the pressure of the gaseous fluid in the valve between its valve seat and its associated one or more orifices tending to urge said valve member in a direction away from its valve seat, the area of said first surface and said third surface each being greater than the area of said second surface.
 8. In apparatus as defined in claim 3; each of said valve members having a first surface subject to the pressure of the gaseous fluid externally of the valves tending to shift said valve members toward its closed positions, a second surface subject to the pressure of the fluid in the tubing string when the valve member is engaged with its seat tending to shift said valve member toward its open position, and a third surface subject to the pressure of the gaseous fluid in the valve between its valve seat and its associated one or more orifices tending to urge said valve member in a direction away from its valve seat, the area of said first surface and third surface each being greater than the area of said second surface; said pilot gas lift valve being constructed and arranged to open at a desired normal difference in pressure between the fluid pressure in the well bore externally of the tubing string and the fluid pressure internally of the tubing string; said main valve being constructed and arranged to open at a pressure differential which is lower than the opening pressure differential of said pilot valve.
 9. In apparatus as defined in claim 3; each of said valve members having a first surface subject to the pressure of the gaSeous fluid externally of the valves tending to shift said valve members toward its closed positions, a second surface subject to the pressure of the fluid in the tubing string when the valve member is engaged with its seat tending to shift said valve member toward its open position, and a third surface subject to the pressure of the gaseous fluid in the valve between its valve seat and its associated one or more orifices tending to urge said valve member in a direction away from its valve seat, the area of said first surface and said third surface each being greater than the area of said second surface; said pilot gas lift valve being constructed and arranged to open at a desired normal difference in pressure between the fluid pressure in the well bore externally of the tubing string and the fluid pressure internally of the tubing string; said main valve being constructed and arranged to open at a pressure differential which is lower than the opening pressure differential of said pilot valve; said main valve being constructed and arranged to close at a desired normal difference in pressure between the fluid pressure in the well bore externally of the tubing string and the fluid pressure internally of the tubing string; said pilot valve being constructed and arranged to close at a pressure differential which is higher than the closing pressure differential of said main valve.
 10. In apparatus as defined in claim 3; wherein said supplemental means comprises spring means.
 11. In apparatus as defined in claim 3; wherein said supplemental means comprises a spring connected to said pilot valve member, and coengageable means on said pilot valve member and main valve member for transmitting the force of said spring to said main valve member when said pilot valve member is in open position.
 12. In apparatus as defined in claim 3; each of said valve members having a first surface subject to the pressure of the gaseous fluid externally of the valves tending to shift said valve members toward closed position, a second surface subject to the pressure of the fluid in the tubing string when the valve member is engaged with its seat tending to shift said valve member toward its open position, and a third surface subject to the pressure of the gaseous fluid in the valve between its valve seat and its associated one or more orifices tending to urge said valve member in a direction away from its valve seat, the area of said first surface and said third surface each being greater than the area of said second surface; said supplemental means comprising spring means.
 13. In apparatus as defined in claim 3; each of said valve members having a first surface subject to the pressure of the gaseous fluid externally of the valves tending to shift said valve members toward closed position, a second surface subject to the pressure of the fluid in the tubing string when the valve member is engaged with its seat tending to shift said valve member toward its open position, and a third surface subject to the pressure of the gaseous fluid in the valve between its valve seat and its associated one or more orifices tending to urge said valve member in a direction away from its valve seat, the area of said first surface and said third surface each being greater than the area of said second surface; said supplemental means comprising a spring connected to said pilot valve member, and coengageable means on said pilot valve member and main valve member for transmitting the force of said spring to said main valve member when said pilot valve member is in open position.
 14. In apparatus as defined in claim 3; said main valve having a check valve seat; a valve element engageable with said check valve seat to prevent fluid from flowing from said tubing string through said main valve passage to the exterior of said main valve; means on one of said valve members engaging said valve element to prevent its engagement with said check valve seat when said main valve member engages its companion seat; and means for eNabling the pressure of fluid in said tubing string to act upon said pilot valve member when said pilot valve member engages said pilot seat.
 15. In apparatus as defined in claim 3; said main valve having a check valve seat; a valve element engageable with said check valve seat to prevent fluid from flowing from said tubing string through said main valve passage to the exterior of said main valve; means on one of said valve members engaging said valve element to prevent its engagement with said check valve seat when said main valve member engages its companion seat; and means for enabling the pressure of fluid in said tubing string to act upon said pilot valve member when said pilot valve member engages said pilot seat, comprising a bypass passageway for fluid around said valve element.
 16. In apparatus as defined in claim 3; said main valve having a check valve seat; a valve element engageable with said check valve seat to prevent fluid from flowing from said tubing string through said main valve passage to the exterior of of said main valve; means on one of said valve members engaging said valve element to prevent its engagement with said check valve seat when said main valve member engages its companion seat; and means for enabling the pressure of fluid in said tubing string to act upon said pilot valve member when said pilot valve member engages said pilot seat, comprising a bypass passage in said main valve member permitting flow of fluid around said valve element to said pilot valve member.
 17. In apparatus as defined in claim 3; said main valve having a check valve seat; a valve element engageable with said check valve seat to prevent fluid from flowing from said tubing string through said main valve passage to the exterior of said main valve; means on said main valve member engaging said valve element to prevent its engagement with said check valve seat when said main valve member engages its companion seat; and means for enabling the pressure of fluid in said tubing string to act upon said pilot valve member when said pilot valve member engages said pilot seat.
 18. In apparatus as defined in claim 3; said main valve having a check valve seat; a valve element engageable with said check valve seat to prevent fluid from flowing from said tubing string through said main valve passage to the exterior of said main valve; means on said main valve member engaging said valve element to prevent its engagement with said check valve seat when said main valve member engages its companion seat; and means for enabling the pressure of fluid in said tubing string to act upon said pilot valve member when said pilot valve member engages said pilot seat, comprising a bypass passage in said main valve member permitting flow of fluid around said valve element to said pilot valve member. 